In the medical field or the sport field, it is desirable to know the distribution of the pressure forces exerted by the feet of a person, in static or dynamic position. In the medical field, a sole with pressure sensors finds applications as a diagnostic sole in podology or orthopedics. Daily worn by a diabetic patient suffering from neuropathy, a sole with pressure sensors may allow improving the prevention of foot lesions. In the sport field, a sole with pressure sensors, worn by a sportsman and connected to a smartphone, allows the sportsman to quantify his running. Analyzing the distribution of the pressure forces applying in particular at the foot sole during walking, running or jumping may allow the sportsman to consciously correct a postural unbalance in order to avoid the occurrence of pains or injuries.
There exist devices with pressure sensors based on optical, magnetic or electrical technologies, and in particular with resistive sensors, inductive sensors (see EP 2607876) or capacitive sensors (see U.S. Pat. No. 7,343,813, US2014/076066).
An advantage of the capacitive pressure sensors is to be little sensitive to the temperature variations.
A capacitive pressure sensor includes at least two electrodes separated by a dielectric material. The electrical capacitance of a capacitive sensor is given by the formula of a capacitor between two plates:
                    C        =                              ɛ            ·            S                    L                                    (        I        )            
where C represents the electrical capacitance of the capacitive sensor, of the capacitor type, S the surface of the electrodes placed opposite to each other, L the distance between the two electrodes and ε the dielectric constant of the material between the electrodes.
Under the effect of a normal pressure force, the variation of thickness L of the dielectric material produces an inversely proportional variation of the electrical capacitance C of the sensor.
It is known for example from the document U.S. Pat. No. 5,449,002 a capacitive pressure sensor based on a resilient polyurethane dielectric in sandwich between two electrical conductors. The variation of the electrical capacitance of this sensor is almost linear as a function of the weight applied, which allows an easy detection. This sensor may be used as a shoe sole, a gripping handle or a support to measure the compressive forces in various medical equipments such as crutch, wheelchair, treadmill. However, such a sensor provides no pressure measurements spatially resolved on the surface of the sensor. Moreover, this capacitive pressure sensor does not allow discriminating a normal pressure from a pressure induced by a shear force.
Now, by application of the formula (I), under the effect of a shear force, a variation of the surface S of the opposite electrodes produces a proportional variation of the electrical capacitance of the sensor. In the case of an elastically deformable dielectric material, the variation of surface S induced by the shear force is generally accompanied with a variation of the thickness L. It is hence necessary to measure independently the variation of thickness to extract from the capacitance variation measurement a measurement of the variation of surface S, in order to deduce therefrom a measurement of the shear force.
Recently, different examples of multiple capacitive sensors have allowed discriminating the normal pressure measurement from the shear force measurement (see US 2013/0093437, U.S. Pat. No. 8,250,926). The capacitive pressure and shear sensors are used in particular in the field of touch screens, haptic interfaces, textiles integrating sensors.
However, the integration of a great number of capacitive pressure sensors to manufacture a high-spatial-resolution sensor network has for drawback to require a far greater number of electrical connections connecting the sensor network to the measurement system.
The document R. Supraneni, Q. Guo, Y. Xie, D. J. Young and C. H. Mastrangelo “A three-axis high-resolution capacitive tactile imager system based on floating comb electrodes”, Journal of Micromechanics and Microengineering, 23 (2013) 075004, describes the design and the manufacturing of a high-spatial-resolution tactile imager for measuring the compressive and shear forces. The tactile imager includes a dielectric formed of a sheet of silicone polymer and a flexible printed circuit (FPCB). Each cell of the tactile imager includes two capacitors sensitive to displacements in a direction X and two capacitors sensitive to displacement in a direction Y. The four capacitors of a cell include, on one face of the dielectric, floating electrodes, and, on the other face of the dielectric, a FPCB supporting comb-shaped electrodes connected by two vertical electrical tracks and two horizontal electrical tracks. The electrical measurement of the four capacitors of a cell requires the multiplexed addressing of the vertical and horizontal electrical tracks, to provide normal pressure measurements in the direction Z, and shear measurements in the directions X and Y. Nevertheless, the horizontal and vertical electrical tracks being deposited on a same printed circuit, the method of manufacturing the printed circuit requires the superimposition of at least two levels of electrical tracks connected by interconnections or vias. A drawback of the method of manufacturing of the dual-level printed circuit is that it requires a greater number of steps of manufacturing. Moreover, this method requires a rigid printed circuit substrate, which may suit for tactile applications, but generally do not suit for an application to a sole with pressure sensors. Finally, the interconnections between two levels of electrodes deposited on a deformable substrate are fragile and may create electrical faults.
Contrary to the tactile applications in which the substrate is generally rigid and may be thick, a sole with pressure sensors must have both a small thickness, lower than a few to millimeters, and a very high flexibility. A sole with pressure sensors must support a pressure dynamics comprised between 0 and 15 kg/cm2.
On the other hand, a drawback of the capacitive sensors based on deformable dielectric materials is that the deformation thereof generally shows hysteresis, liable to induce measurement errors.
Moreover, the capacitive shear-force sensors have a lower sensitivity than the normal pressure sensors. A capacitive shear-force sensor must generally extend over a greater surface than a capacitive normal-pressure sensor.
There thus exists a need for a system and a method allowing manufacturing a sensor network system for measuring pressure forces with a high spatial resolution, while having a small thickness and a great flexibility, in particular for the application to a sole with pressure sensors.
One of the objects of the invention is to propose a sensor network system for measuring pressure forces with a high spatial resolution, having a small thickness and a limited number of electrical connections. Another object of the invention is to provide not only compressive force measurements, but also shear force measurements.
Another object of the invention is to propose a sensor network system that suits for soles in the medical, sport or entertainment field.
Another object of the invention is to propose a simple, rapid and cheap method of manufacturing, allowing manufacturing a high-resolution capacitive sensor network system.